1. Field of the Invention
The present invention relates to a communication processor and, more specifically, to a one chip communication processor for carrying out protocol processing of a plurality of layers in a hierarchical network system in which communication functions are provided in a plurality of hierarchical layers.
2. Description of the Background Art
OSI (Open Systems Interconnection) represents international standard of a network architecture, the standard of which is now being developed mainly by ISO (International Organization for Standardization). The object of OSI is to enable free exchange of data among terminals and computers of different types.
In order to enable mutual communication of arbitrary application processes in a computer network, rules with respect to data transfer via repeater open system, conversation between application processes, access to various resources (files and the like) and so on must be defined as protocols, based on data transfer between open systems directly coupled by means of physical medium. Further, it is necessary to make these protocols available by appropriately combining the same in accordance with field of application and structures of the computer network.
Therefore, in a basic reference model of the OSI, communication functions of the open system including function of controlling communication circuits and a series of communication processing functions (encryption, code conversion, display control, file transfer.accessing, data base accessing and the like) are divided into seven functional hierarchical layers which operate in order corresponding to processes of communication (see FIG. 6). Necessary protocols are determined for every stage of the functional hierarchy so as to realize the function allotted to that layer. This is the concept of protocol hierarchy, and each functional layer is called as a protocol layer or, simply, a layer.
By this protocol hierarchy, a protocol for realizing the communication function allotted to each layer can be closed in that layer, thereby maximizing independency of the protocol in respective layers. Consequently, influence of introduction of new technology, influence derived from expansion.modification of communication functions incidental to newly generated needs of users can be limited to layers which are directly related. Therefore, maintainability of protocols and protocol products can be improved. Each layer may be divided into sub layers, as needed.
Seven layers shown in FIG. 6 will be briefly described.
(1) Physical Layer (first layer)
Various physical media such as communication satellites and optical fibers are used for data transmission. The physical layer provides function of transmitting "bit strings" through such physical media. More specifically, physical conditions, electrical conditions and the like necessary for bit transmission are defined.
(2) Data Link Layer (second layer)
By using bit transmitting function of the physical layer, transparent and highly reliable data transmission is carried out between adjacent systems (nodes). This layer corresponds to a conventional high level transmission control procedure (HDLC) and it includes data transmission error control procedure and the like.
(3) Network Layer (third layer)
Exchange of data between systems at both ends is enabled through various communication nets such as public packet switching network. For this purpose, a routing function for designating communication path to transmitting data to the destination and function of passing data (data relay function) among nodes are necessary.
(4) Transport Layer (fourth layer)
Transparent and highly reliable data transfer is carried out between processes of systems at both ends. If quality of services provided by the lower network layer does not satisfy the service quality (for example, rate of data transmission error) required by the application, the transport layer compensates for the lack of service.
(5) Session Layer (fifth layer)
In order to realize efficient conversation between application processes at both ends, this layer provides synchronization, selection of transmission mode and controls priority of transmission.
(6) Presentation Layer (sixth layer)
This layer controls forms of data and the like so that data treated by the application process can be accurately and efficiently transferred.
(7) Application Layer (seventh layer)
In this layer, application service elements such as file transfer, access and management (FTAM), message handling processing system (MHS), virtual terminal (VT) and remote data base access (RDA) are carried out corresponding to various applications executed by the user.
Integrated Services Digital Network (hereinafter referred to as ISDN) is known as one of the data networks employing the seven layered model of OSI described above. ISDN digitally integrates transmission paths and switching devices so as to provide, in united manner, various communication services such as telephone, data and images. In the conventional communication system prior to the ISDN, an interface between a terminal equipment of a user and a communication network is adapted to a fixed use such as for telephone or for data communication. However, in the ISDN, an integrated interface for the above mentioned various services is defined. This interface is referred to as general purpose user net interface, which is definitely defined by International Telegraph and Telephone Consultative Committee (hereinafter referred to as CCITT).
An application example of the ISDN is shown in FIG. 7. Referring to FIG. 7, in the ISDN, an ISDN switching device 18 provided in telephone exchange is connected to an ISDN terminal (TE) of a user by a telephone line 28. A network terminal (NT) 100 is provided at the user's residence, and telephone line 28 and four-wire bus 19 are connected thereto. Each ISDN terminal is connected through the four-wire bus 19 to the network terminal 100. A transmitting/receiving apparatus 27 for transmitting and receiving data signals through four-wire bus 19 is provided as an interface circuit at an input/output portion of each of the network terminal 100 and ISDN terminal.
FIG. 8 is a schematic block diagram of a telephone 20 as an example of the ISDN terminal. Referring to FIG. 8, the telephone 20 includes a transmitting/receiving apparatus 27 connected through a transformer 29 for signal transmission/reception to four-wire bus 19; a second layer processing apparatus 22 for realizing the functions of the second layer of the ISDN basic interface; a third layer processing apparatus 23 for realizing the functions of the third layer of the ISDN basic interface; a key pad 24; a CODEC (coder decoder) apparatus 25 for coding/decoding audio signals; a handset 26; and a switch 30.
The transmitting/receiving apparatus 27 is provided for realizing the function of the first layer of the ISDN basic interface. The four-wire bus 19 includes a transmission bus 19a for transmitting signals from telephone 20, and a receiving bus 19b for transmitting signals applied to telephone 20.
At call incoming, the reception signal transmitted through the receiving bus 19b is received by the transmitting/receiving apparatus 27 through transformer 29. The received signal is subjected to processing such as error detection in the second layer processing apparatus 22, and then it is applied to third layer processing apparatus 23. Third layer processing apparatus 23 identifies the telephone number and the contents of the requested service. While telephone 20 is being called, the third layer processing apparatus 23 turns on switch 30 to provide a communication path between transmitting/receiving apparatus 27 and CODEC apparatus 25. Consequently, CODEC apparatus 25 receives the signal which has been received by the transmitting/receiving apparatus 27, converts the same to an audio signal, and applies the converted audio signal to handset 26.
In calling, necessary information such as telephone number is applied through key pad 24 to the third layer processing apparatus 23. Third layer processing apparatus 23 converts the given information to data of a prescribed format and applies the same to second layer processing apparatus 22. Second layer processing apparatus 22 adds information such as error correction to the applied data, and transmits the data to transmitting/receiving apparatus 27. Transmitting/receiving apparatus 27 provides the applied data to transmission bus 19a through transformer 29. When a communication permission signal is returned from the other party in response to the transmitted data from telephone 20, the permission signal is received through transformer 29 by transmitting/receiving apparatus 27. The received signal is subjected to processings such as error correction by second layer processing apparatus 22, and then it is applied to third layer processing apparatus 23. Third layer processing apparatus 23 recognizes the communication permission signal, and turns on switch 30. Thus a communication path is established between transmitting/receiving apparatus 27 and CODEC apparatus 25. The audio signal generated from handset 26 is converted to a digital audio signal by means of CODEC apparatus 25. The converted digital audio signal is applied to transmitting/receiving apparatus 27 through switch 30. Transmitting/receiving apparatus 27 supplies the converted digital audio signal to transmission bus 19a through transformer 29, as a transmission signal.
FIG. 9 is a block diagram showing more detailed structure of second layer processing apparatus 22 and third layer processing apparatus 23 shown in FIG. 8. The second layer processing apparatus 22 includes a communication data converter 1; an FIFO (First-In First-Out) 2 for data reception; an FIFO 3 for data transmission; an internal bus 4; a signal line 5 connecting communication data converter 1 with transmitting/receiving apparatus 27 (see FIG. 8); a processing circuit 6 formed of, for example, a CPU; a DAMC (Direct Memory Access Controller) 7; a bus interface 8; and a ROM 9 storing instructions for the second layer protocol processing. Generally, second layer processing apparatus 22 is formed by one chip. Meanwhile, third layer processing apparatus 23 includes a ROM 10 storing instructions for third layer protocol processing; and a microprocessor 12 for carrying out the protocol processing of the third layer. Since ROM 10 has larger storage capacity than ROM 9, ROM 10 is generally mounted on a chip different from microprocessor 12. Second layer processing apparatus 22 and third layer processing apparatus 23 are connected to a system bus 13 so that data can be transmitted to and from each other. An external RAM 11 is further connected to system bus 13.
The operation of the conventional apparatus shown in FIG. 9 will be described. In second layer processing apparatus 22, a signal received from first layer processing apparatus (transmitting/receiving apparatus 27) through signal line 5 is input to communication data converter 1. Communication data converter 1 carries out flag detection, zero deletion, and frame error detection. An output data from communication data converter 1 is stored in the FIFO 2 for reception. Processing circuit 6 successively takes out data stored in FIFO 2 for reception, and carries out protocol processing of the second layer in accordance with instructions read from ROM 9. Remaining data related to the third and upper layers are transmitted to external RAM 11 through bus interface 8 and system bus 13 under control of DMAC 7. Microprocessor 12 in third layer processing apparatus 23 successively takes out data stored in external RAM 11, and carries out protocol processing of the third layer in accordance with instructions read from ROM 10.
The conventional communication processing apparatus is structured as described above. A dedicated processing apparatus (for example, transmitting/receiving apparatus 27, the second layer processing apparatus 22 and third layer processing apparatus 23 of FIG. 8) is provided for each of the layers. However, the processing apparatuses of the respective layers do not always operate constantly from the start to the end of the communication. For example, at the start of communication when link connection is established by the second layer processing apparatus, the third layer processing apparatus does not operate, since processing in the third layer takes place after the end of processing in the second layer. Therefore, hardware of processing apparatuses of respective layers was not effectively used in the conventional communication processing apparatus.
In the conventional communication apparatus, an external memory device (for example, external RAM 11 provided between second layer processing apparatus 22 and third layer processing apparatus 23 of FIG. 9) was necessary for exchange of data between two adjacent layers. Such external memory devices are provided between respective layers, increasing memory capacity of the whole system.
In addition, in the conventional communication processing apparatus, data transmission path (for example system bus 13 of FIG. 9) between respective layers must be frequently used because of data communication between respective layers.